The present invention relates to a rotary tube for a rotary tube furnace as per the preamble of claim 1 and a rotary tube furnace having such a rotary tube. Furthermore, the present invention relates to the use of such a rotary tube or rotary tube furnace for producing activated carbon.
Activated carbon is the most widely used adsorbent because of its quite unspecific adsorptive properties. Legal requirements, but also increasing environmental consciousness, are leading to an increased demand for activated carbon.
Here, activated carbon is increasingly employed both in the civilian sector and in the military sector. In the civilian sector, activated carbon is employed, for example, for the purification of gases, filter units for air conditioning and automobile filters, while in the military sector activated carbon is used, for example, in protective materials of all types (e.g. breathing protection masks, protective coverings and pieces of protective clothing of all types, e.g. protective suits).
Activated carbon is generally obtained by sulfonation, carbonization (synonymously also referred to as low-temperature carbonization, pyrolysis or coking) and subsequent activation of suitable carbon-containing starting materials. Here, preference is given to starting materials which lead to economically feasible yields. This is because elimination of volatile constituents during carbonization and as a result of burning during activation lead to considerable weight losses. For further details on the production of activated carbon, reference may be made, for example, to the publication H. v. Kiente and E. Bäder “Aktivkohle and ihre industrielle Anwendung”, Enke Verlag Stuttgart, 1980.
The nature of the activated carbon produced, fine- or coarse-pore, strong or brittle, etc., depends on the carbon-containing starting material. Customary starting materials are, for example, coconut shells, wood scrap, peat, hard coal, pitch, or else particular plastics such as polymers which play an important role in, inter alia, the production of activated carbon in the form of grains or spheres.
Activated carbon is used in a variety of forms: powdered carbon, crushed carbon, shaped carbon and since the end of the 1970s also granular and spherical activated carbon (known as “granulated carbon” or “spherical carbon”). Granular, in particular spherical, activated carbon has a series of advantages over other forms of activated carbon which make it valuable or even indispensable for particular applications: it is free-flowing, tremendously abrasion-resistant, dust-free and very hard. Granular carbon, in particular spherical carbon, is very sought-after for particular fields of use, e.g. sheet-like filter materials for protective suits to protect against chemical poisons or filters for low pollutant concentrations in large amounts of air, because of its specific shape but also because of its extremely high abrasion resistance.
In the production of activated carbon, in particular granular carbon and spherical carbon, suitable polymers are in most cases used as starting materials. Preference is given to using polymers, in particular divinylbenzene-crosslinked styrene polymers. For example, the precursors of ion-exchange resins (i.e. unsulfonated ion-exchange resins), which are usually divinylbenzene-crosslinked polystyrene resins, serve as suitable starting material. This starting material is then typically sulfonated in-situ in the presence of sulfuric acid or oleum.
However, it is also possible to use ion-exchange resins (e.g. cation-exchange resins or acidic ion-exchange resins, preferably having sulfonic acid groups, e.g. cation-exchange resins based on sulfonated styrene-divinylbenzene copolymers). No subsequent sulfonation is carried out in the case of this starting material.
Sulfonation is a reaction in which a sulfonic acid group or sulfo group is introduced into an organic compound. The reaction products are referred to as sulfonic acids. In the case of finished ion exchangers, the sulfonic acid groups are already present in the material, while in the case of the ion exchanger precursors they have to be introduced by sulfonation. The sulfonic acid groups perform a critical function since they play the role of a crosslinker by being eliminated during carbonization. However, the large amounts of sulfur dioxide liberated and the associated corrosion problems in the production apparatuses are disadvantageous and problematical.
The production of activated carbon is usually carried out in rotary tube furnaces. These have, for example, an inlet opening for charging with a feed material and for introducing gases and also an output opening for taking out the end product and for discharging gases. The production of activated carbon in rotary tube furnaces can be carried out in a continuous or batch process.
In the carbonization, which can be preceded by a precarbonization or low-temperature carbonization phase, the carbon-containing starting material is converted into carbon, i.e. in other words the starting material is carbonized. In the carbonization of the above-mentioned organic polymers based on styrene and divinylbenzene, the functional chemical groups, in particular sulfonic acid groups, are destroyed with elimination of volatile constituents, in particular SO2, and free radicals, which have a strong crosslinking action, are formed without there being a pyrolysis residue (=carbon). The organic polymers contain crosslinking functional chemical groups (in particular sulfonic acid groups) which on thermal decomposition lead to free radicals and thus to crosslinks.
In general, the carbonization is carried out under an inert atmosphere (e.g. nitrogen) or at most a slightly oxidizing atmosphere. It can equally well be advantageous to add a small amount of oxygen, in particular in the form of air (e.g. from 1 to 5%), to the inert atmosphere during carbonization, in particular at relatively high temperatures (e.g. in the range from about 500° C. to 650° C.), in order to bring about oxidation of the carbonized polymer skeleton and in this way aid subsequent activation.
Owing to the acidic reaction products (e.g. SO2) eliminated during carbonization, this stage of the production process for the activated carbon is extremely corrosive in respect of the material of the rotary tube or rotary tube furnace and places severe demands on the corrosion resistance of the material of the rotary tube or rotary tube furnace.
The carbonization is then followed by activation of the carbonized starting material. The basic principle of activation is to degrade, selectively and in a targeted manner, part of the carbon generated in the carbonization under suitable conditions. This forms additional pores, clefts and cracks and the surface area per unit mass of the activated carbon increases considerably. Thus, a targeted burning of the carbon is carried out during activation. Since carbon is reacted during activation, a sometimes considerable loss of material occurs during this operation, and this is, under optimal conditions, equivalent to an increase in the porosity and produces an increase in the internal surface area (pore volume) of the activated carbon. The activation is therefore carried out under selective or controlled oxidizing conditions.
Customary activating gases are, in general, oxygen, especially in the form of air, water vapor and/or carbon dioxide and also mixtures of these activating gases. Inert gases (e.g. nitrogen) can optionally be added to the activated gases. In order to achieve an industrially satisfactory high reaction rate, the activation is generally carried out at relatively high temperatures, in particular in the temperature range from 700° C. to 1200° C., preferably from 800° C. to 1100° C. This places great demands on the temperature resistance of the material of the rotary tube.
The various process steps, namely sulfonation, carbonization and activation, place different, severe demands on the material of the rotary tube. Particularly when all three process steps are to be carried out batchwise in a single rotary tube, the material of the rotary tube has to withstand
a) the highly corrosive conditions during sulfonation,
b) the very corrosive conditions during carbonization and
c) the high-temperature conditions during activation.
For this reason, only materials which combine good resistance to chemically aggressive materials, in particular a high resistance to acids, a good corrosion resistance and a good high-temperature stability in one material are used for producing the rotary tube.
A further requirement which the rotary tube has to meet results from the need for homogeneous contacting of the feed material, in particular the sulfonated, carbonized starting materials, with the activating gases. For this reason, means for intimate mixing of the feed material are provided. Without sufficient mixing of the feed material during rotation of the rotary tube, a significant part of the feed material rests against an interior of the rotary tube during rotation of the rotary tube or the individual particles of the feed material rest on one another and are rotated with the rotary tube until they finally drop downward. As a result, both the contact area and the contact time of the feed material with the activating gases are very low. This results in poorer quality activated carbon, namely an activated carbon having a low degree of activation, in particular having a less porous structure and a lower internal active surface area. Furthermore, unsatisfactory mixing of the feed material increases the time required for producing the activated carbon, in particular the activation time.
A rotary tube which satisfies the above-mentioned requirements is disclosed in DE 10 2004 036 109 A1, from which the present invention proceeds. The known rotary tube is provided for a rotary tube furnace and is configured for the production of activated carbon by means of sulfonation, carbonization and activation in a batch process. The rotary tube has a rotary tube body and at least one mixing section for mixing a feed material.
The mixing section of the known rotary tube has a plurality of mixing elements having fastening sections. The fastening sections of the mixing elements are pushed in through openings in the rotary tube body and welded on the outside of the rotary tube body. In the known rotary tube, it was recognized that mixing elements welded on the inside to an inside of the rotary tube are problematical since welding of the mixing elements can result in embrittlement of the material and since the welding seams in the interior of the rotary tube, which are subjected to severe stresses during activated carbon production, require continual maintenance and checking outside operation of the rotary tube.
Although the rotary tube body and the mixing elements of the known rotary tube consist of high-temperature-resistant and corrosion-resistant, high-alloy steel, abrasion of metals from the high-alloy steel can occur during rotation of the known steel rotary tube as a result of sliding of the feed material over the inside of the rotary tube body and over the mixing elements. This metal abrasion leads to contamination of the activated carbon. This shows up in, for example, an increased iron content in the activated carbon.
However, to meet high-purity requirements, for example in the pharmaceutical sector, it is necessary to have activated carbon which has no such impurities, in particular no iron-containing impurities or other traces of metal originating from the steel of the rotary tube.